Sunday, September 25, 2011

San Francisco Green Flash

This was half written a week or so ago but things have been far too busy to post it until today. I'm now in Warsaw, getting ready for a three day Marie Skłodowska event. I took a stroll around the city this morning and caught up on some papers in a cafe before coming back to get ready for the events which start this afternoon. Anyway, here is the finished post from last week:


A non-stop week, but I've ducked out of a dinner tonight to catch up on emails which I haven't had a chance to go through for a few days, and to update a few details from the SF trip.

The first few days of the holiday were spent in San Francisco, relaxing and reading in cafes and working out what the next days should hold. I'd been to San Francisco a few times before, the previous two for work at SLAC and hadn't had much chance to explore the city so this was a good, relaxed few days taking in the atmosphere.

I headed out to the piers to find the Golden Gate Bridge, only to remember that it couldn't be seen from there. The fishermen there made for some good compositions though so I took advantage of the contrasts and spent some time chatting with the Chinese fishermen.
fishermen in San Francisco
fishermen in San Francisco
I did at least manage to get the bay bridge:
fishing rods over the bay bridge
In fact by far the most exciting photo opportunity for me came towards the end of the first week, when I was couchsurfing in Ocean Beach, an area that I'd never explored previously. It turned out to have a completely different atmosphere from the rest of SF that I know and it's even more laid back than the rest of what is known to be a pretty chilled place. The weather was perfect and I'd scouted out the line on the beach previously from where I was hoping to catch the sunset. I headed there with my couchsurfing host and sat on top of a sand dune watching the sun descend. The sky was clear, but the convection currents of rising warm air made for layers in the sky above the ocean of more and less dense air. These different layers, called temperature inversions because the temperature does not go linearly down with altitude, cause the sun's rays to be refracted in complex ways and force the otherwise circular image to be distorted as it sets. These are the perfect conditions for green flash sunsets. Before it had gotten into its final stages of descent a flock of birds flew past, signalling the beginning of what was to be an exciting atmospheric optics session:
sunset and bird formation
As the sun set further, the distortions grew stronger and, because of the uneven nature of the temperature inversions, you can get some strange, lop-sided images:
sunset distortion
The green and red light from the sun naturally get refracted by different amounts as they travel through the atmosphere and the temperature inversion layers act to magnify the effect. As the top of the sun passed through the boundary between two layers the green light was momentarily magnified and caused a brief mock-mirage green flash:
green flash sunset
As it set further the lower reaches of the sun can appear to be pulled from the rest and you see a strange, hugely distorted second image of the sun on the horizon. Really this is a mirage of the sun above. Another disconnection in this one had also appeared above the sun:
Further down and more distortion, with hints of green around the upper limb of the sun, birds accompanying the shot:
sunset with hints of green flash
By a lovely coincidence the one boat that was on the horizon, also enhanced by a mirage effect was exactly in the setting sun's path
sunset distortion
and stayed there until the sun made its final warped movements:
sunset and ship
I took around 50 photos during the sunset, and the above, to me were the most interesting. I've seen perhaps five or six green flash sunsets now and every one has been completely different. As I tell people regularly, these effects are not rare, you just have to know when and where to look. On that note, of course one has to be extremely careful and even at its lowest level I will only ever glance in the direction of the sun momentarily and never stare through the viewfinder. This is the perfect time to use liveview on the camera, but one has to be very careful not to damage the lens or sensor of the camera. The best hint I think is that if you are going to try and take these sorts of photos, only let the camera focus there for the briefest time possible. For more information, as always, go to the fantastic Atmospheric Optics website.

Anyway, from San Francisco I went on a three day trip to Yosemite, but these stories and pictures will have to wait...


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